Prior art heat sealing machines used movable endless metal bands to press together the portions of heat sealable material to be sealed. A typical construction was to use a steam heated or electrically heated stationary block frictionally engaging the rear of the run of endless metal belt at the sealing zone. This passed heat to the metal band and then the metal band passed heat to the material to be sealed. In another prior art construction electrically heated blocks were linked together like a caterpillar tractor tread and an endless metal belt encircled this caterpillar tread to be heated by the electrically heated blocks. Such systems were inefficient in transfer of heat and also had a large heat storage capacity so that they could not respond quickly to required changes in heat. Many electrical heaters were the sheathed type wherein an electrical resistance wire is separated from a tubular metal sheath by electrical insulation. This electrical insulation is also generally a heat insulation so that the transfer of heat from the inner electrical resistance wire to the outer tubular metal sheath was not a rapid process. Next the heat had to be transferred from the tubular sheath to the heater blocks to heat the large mass of such block. Next the heat had to be transferred from the heater block to the rear of the endless metal band. The lack of good surface contact therebetween retarded the heat flow. One result was that the entire sealing machine then became quite hot and personnel had to be shielded from all this excess heat. Another defect was the slowness of operation of the machines. Where the sealing machine was used in the construction of bags to seal one end of the bag, a production rate of 60 bags per minute was a usual speed.
A prime difficulty with such prior art sealing mechanisms was the high wear factor developed between the bands and the heater blocks. In one prior art bag making machine the wear was excessive both on the metal band and on the heater blocks. The heater blocks might last as much as one to three months but often would last only one week to ten days. The wear on the metal band was even more excessive and the band might last for one week but might last only a few hours. Even more of a problem was the excessive downtime of the machine while the band was being replaced or the heater blocks were being replaced or repaired. Still another problem was to attempt to accurately determine when a particular heater in a heater block had burned out. These heaters often burned out at the rate of three or four a week.
Another type of prior art sealing machine used stationary heater bars with the bags or other heat sealable material sliding along in contact with these heater bars. It was found that with the bags sliding on the heater bars one could not use much pressure of the bar against the bag otherwise the machine did not function properly, and without much pressure there was not much heat transfer so that an excessively long machine might be required in order to transfer enough heat into the bag to perform a sealing operation. Where the bars were kept only in close proximity to the bag without touching, for example, a one-eighth of an inch spacing, then a heating machine sixty feet in length was required in order to impart enough heat to the bags, and even then only about sixty bags per minute could be processed by the machine.